In the production of cement clinker, about 0.53 kg of CO2/kg of clinker are formed in the calcination of the raw meal and about 0.26-0.29 kg of CO2/kg of clinker are formed in the combustion of the fuel required. These amounts of CO2 are usually emitted into the atmosphere. Depending on the number of cyclone preheating stages and the radiation and convection losses, the specific fuel energy requirement is 2800-3200 kJ/kg of clinker. For this reason, processes in which the CO2 formed is substantially concentrated in the exhaust gas and the CO2 is subsequently liquefied and stored for the long term in suitable geological formations or is utilized for further purposes have been discussed.
DE 10 2008 023 899 B4 describes a process for reducing the CO2 emission in the production of cement clinker, in which a lime-rich material component and a low-lime material component are preheated in separate preheaters, calcined in a joint calciner and subsequently fired in a furnace to give cement clinker. Here, the exhaust gases from the calciner are utilized in the preheater for preheating the lime-rich material component and the exhaust gases from the furnace are utilized in the preheater for preheating the low-lime material component. Furthermore, fuel and pure oxygen are introduced for combustion into the calciner so as to form a calciner exhaust gas which contains predominantly carbon dioxide and water vapor. The exhaust gas from the preheater for preheating the lime-rich material component is at least partly recirculated as circulation gas to the calciner in order to be introduced there as carrier gas.
WO 2008/059378 A2 discloses a process for producing cement in which the raw meal is firstly preheated by means of exhaust gas from the rotary tube furnace (1100-1300° C.) and is subsequently mostly calcined separated from the furnace exhaust gas on the gas side by means of oxy-fuel firing in the calciner. In oxy-fuel firing, oxygen is used as oxidant instead of air; the oxygen can be produced by means of one of the processes discussed for oxygen production (cryogenic, adsorptive, membrane processes). A CO2-rich calciner exhaust gas is produced in this way. The raw meal is separated out from the oxy-fuel flue gas by means of a cyclone and goes into the rotary tube furnace. The calciner exhaust gas has to be cooled in order for a substream subsequently to be recirculated to the oxy-fuel combustion. During cooling of the flue gas, process steam or steam for power generation, for example, can be produced. However, such utilization of the calciner exhaust gas results in a significantly increased fuel energy requirement for cement production since the exhaust gas is in this case not utilized for preheating the raw meal. Compared to conventional cement production, the fuel energy requirement alone would possibly increase by up to 70% to above 5000 kJ/kg of clinker. In addition, industrial practice shows that a heat exchanger is very difficult to operate at gas entry temperatures of 850-950° C. and at the same time high dust loadings of some hundreds of grams per cubic meter: the dust adheres strongly and is very difficult to clean off because of the high temperatures. Conventional cleaning methods fail here because of the high temperatures and the resulting weakening of materials therefrom.
WO 2010/046345 proposes that a calciner operating according to the stationary fluidized bed principle be used instead of a calciner configured as entrained flow reactor. Since no pneumatic transport of the raw meal within the calciner is therefore necessary, this concept promises to make do with no or only very little flue gas recirculation. It is therefore necessary to cool and recirculate no or only very little flue gas and the exhaust gas can be utilized as usual for preheating the raw material. The fuel energy requirement in this process is therefore no higher or only slightly higher than in the case of the classical production of cement using a preheater oven.
As a result of configuration of the calciner as stationary fluidized bed, significantly less precalcined hot meal gets into the oxy-fuel flue gas path to be cooled. Possible recarbonating effects can be minimized in this way. The heat from the oxy-fuel flue gas is used as heat source for a post-combustion unit for absorption of CO2 from the furnace exhaust gas and subsequent desorption of the relatively pure CO2 from the scrubbing solution. In addition, the waste heat from the exhaust air stream from the cooler is employed here as additional heat source for the desorption.
However, a detailed examination of the fluidized bed concept shows that a fluidized-bed calciner would require very large cross-sectional areas despite pure oxygen operation since in the case of the typically very fine raw meal a stationary fluidized bed can no longer be established even at very low gas velocities. The advantage of the low recarbonating would therefore no longer be present and the disadvantages of the fluidized bed in the form of very large construction volumes and high pressure drops would predominate.